This invention relates to coated electroluminescent phosphors which are resistant to moisture and are relatively chemically inert. More particularly, the phosphors are coated with an aluminum oxide coating which is non-particulate and conformational in nature.
The electroluminescence of powdered zinc sulfide phosphors embedded in an insulating medium and excited by an alternating electric field usually deteriorates from initial brightness in a near hyperbolical curve. Half life is reached when the brightness versus time curve crosses a horizontal line drawn at half the initial brightness.
Deterioration in zinc sulfide electroluminescent phosphors is thought to occur because of a diffusion of copper from the defects which permeate the zinc sulfide crystals. This diffusion causes an increase in electrical resistance along the imperfection lines and a resulting decrease in brightness.
The diffusion of copper occurs because of the presence of sulfide ion vacancies generated by an electrolytic decomposition of the zinc sulfide surface in the presence of water and an electric field.
To increase the half-life of electroluminescent phosphors it is necessary to impede the diffusivity of the copper atoms.